1. Field of the Invention
The present invention relates to an image color correction method and an image forming apparatus, which corrects the color of an image formed by an image forming apparatus. As such image forming apparatus, there can be cited, for example, a copying machine, a printer, a facsimile, and the like which form an image by means of an electrophotographic system or an inkjet system.
2. Related Background Art
In recent years, in general, an image printing technique using an electrophotographic system or an inkjet system has been widely distributed, and attempts at day-to-day improvements of the printing quality and the printing speed have been made.
For example, in the inkjet system, if high-grade media (recording media treated with special processing on the surface) are used, a level not inferior to a photographic paper photograph is reached. Further, in the electrophotographic system, owing to improvement of printing speed and expansion of the color reproduction area, a near-print-quality market has arisen. In offset printing, in addition to cyan (C), magenta (M), yellow (Y), and black (K), inks of red (R), green (G), blue (B), and grey (light color of black) are adapted not in a solid but in a half tone, thereby improving the expressive power of color.
While attempts to improve the printing quality itself have been made in this way, in whichever system is used, as described above, there is a problem of stability of printing quality. Here, among the problems that get in the way of achieving the desired stability of printing quality, are instability of the density and color of individual images at the time of continuous printing (“continuous printing fluctuation”), change of the density and color of the output image after a lapse of time (“time variability”), difference in the density and color of the outputted image owing to fluctuation of the environment in which the image forming apparatuses of the inkjet system and the electrophotographic system are installed (“environmental fluctuation”), difference in the density and color of the image before and after being left alone in a case in which the image forming apparatus is left alone in a power-off state for a long period of time (“long-term fluctuation”), and the like.
For the problems described above, a short-term fluctuation of the density and color of the image owing to the continuous printing fluctuation and the environmental fluctuation can be stabilized to a certain extent by means of a calibration mechanism provided in each apparatus. In the meantime, the long-term fluctuation often exceeds what can be corrected by each apparatus, and it is difficult to perform the calibration automatically.
Hence, to stabilize the long-term fluctuation, there is known an image forming apparatus that reads a predetermined gradation test pattern formed by a copying machine through reading means of the copying machine, and uses density data of each gradation level of the read gradation test pattern, to perform a gradation correction (see Japanese Patent Publication No. 3,276,744).
Further, in U.S. Pat. No. 6,215,562, there is disclosed a technique for performing a color adjustment of the printer at a low cost without using reading means of a scanner, a reader and the like.
Further, as a technique for performing a color adjustment of a printer at a low cost without using reading means of a scanner, a reader and the like, there is available a product under the trademark Visual Cal (registered trademark), which is a calibration function, commercialized by EFI (registered trademark) Electronics Corporation. This can visually make a color adjustment without using an expensive concentration meter, just by the operation of the device's main-body control panel. To be specific, a gradation pattern is formed on a sheet under a predetermined condition, and at the same time, a comparison pattern to be compared is formed in its vicinity, and an operator compares the density of these patterns, so that an image forming condition in the image forming apparatus is corrected.
However, according to the technique disclosed in Japanese Patent Publication No. 3,276,744, reading means such as a scanner, a reader and the like is required, but this is not adaptable to the image forming apparatus not mounted with the reading means such as a printer and the like.
In the meantime, according to a technique disclosed in U.S. Pat. No. 6,215,562, there is a problem in that the above described “pattern to be compared” is formed with a smaller number of lines (so, lower resolution) than in normal image formation, in consideration of the stability of the image density; even an experienced operator may find it difficult to compare and adjust an actual image density. Further, when the density of the gradation pattern formed on the sheet and the comparison pattern are compared, since the patterns are affected by external light such as a fluorescent lamp, a spot light and the like, the comparison of the density of the gradation pattern and the comparison pattern is not performed adequately, thereby causing a correction defect of the image density.
Further, since this method has to use a prescribed paper as the paper on which to form the pattern to be compared with the gradation pattern, no countermeasure can be taken unless this prescribed paper is available.